Nitrogen-purged storage desiccators provide a clean, dry storage environment for stored, moisture-sensitive items. Configured for use with a nitrogen purge gas flow controller, such as one sold by Terra Universal, Inc. of Fullerton, Calif., under the name ISODRY™, desiccators provide a continuous purge of clean, dry nitrogen to flush out moisture-laden air.
For decades, desiccator storage has been a common practice in many industries, including semiconductor, electronics, aerospace and medical device manufacturing. As critical components become smaller and more sophisticated, their susceptibility to moisture damage increases. In recent years, desiccators have been widely used in bio-pharmaceutical manufacturing to inhibit moisture-related degradation of drugs and biological samples. Once absorbed by sensitive components, water creates a number of potentially disastrous conditions with costly effects. Even minute traces of oxidation, the most notorious result of moisture exposure, can degrade soldering and other manufacturing processes. Because water dissolves ionic contaminants, it also alters the conductivity of the material, which in turn can degrade electrical function. Water also combines with other materials, causing harmful chemical reactions that degrade pharmaceutical samples and chemical mixtures.
One common method of dealing with moisture contamination is to remove it prior to each manufacturing step. Although vacuum processing and bake-and-bag methods of sample drying accomplish this end, these operations slow down production, particularly if they must be repeated several times in the course of circuit manufacturing. Further, these baking and sealing processes themselves expose parts to thermal extremes that can cause damage.
Desiccant dryers avoid some of these drawbacks, but introduce others. Such desiccant dryers remove moisture from air (or other process gas) inside the desiccator chamber and often feature dual module designs that perform online drying and offline desiccant regeneration simultaneously for continuous operation. Such dryers can be effective, but they require heating/drying components that may not be reliable or that may affect stored components. It can take many hours to reduce ambient conditions to a relative humidity of ten percent water vapor at room temperature. Their complexity and high operating costs makes them prohibitively expensive for long-term storage applications.
As an alternative to desiccant dryers, nitrogen-purged desiccator systems maintain dry conditions relatively cheaply and conveniently. The fundamental principle of nitrogen-purged desiccator cabinets is to displace moist air with nitrogen gas. Such systems employ one or more chambers in which the moisture-sensitive content is stored. A continuous purge of nitrogen gas continuously enters the cabinet, displacing any water vapor, and exits through an exhaust valve preferably on a side opposite a gas inlet. Displacement (sweeping), which depends on laminar flow, is the common method employed, although some mixing may be achieved. Consequently, the current technologies discussed above fail to achieve laminar flow effectively. The concept, design, and construction of conventional nitrogen purge desiccators thus incorporate conflicting technologies of displacement and mixing, resulting in inefficiencies.
Figure A depicts a prior art desiccator a having a single displacement channel DC, where dry nitrogen gas N2 is released into the highest chamber 1, to be distributed in a downward flow of gas through the stacked-up vertical chambers 1-5. This is the most common and believed to be the most economical configuration for desiccators. The chambers 1-5 are connected in series and in communication with one another through perforations P in the floors FL of the chambers 1-4, so gas flows in series from a higher chamber into a lower chamber. The nitrogen gas N2 entering the highest chamber 1 moves downward mixing with the moist air in the chambers 1-4. The most basic configuration utilizes a manually-adjusted gas inlet connected to a flow meter FM. Upgraded models of the flow meter FM use an automatically controlled humidity module in which the user can specify a predetermined humidity set point as a percent of water vapor (RH %). When a humidity sensor (not shown) detects a moisture reading above the set point, a solenoid (not shown) opens a valve to release purge gas until the relative humidity set point is reached.
As shown in Figure A, the dry nitrogen gas N2 is flushed into the series of chambers 1-5 from a single point inlet I to displace humid air through a single point outlet 0. The chambers 1-5 are in communication with each other so gas flows in series from one chamber to the next. The graph of FIG. 26 shows the performance and concentrations of moisture within each chamber of Figure A. As shown in the graph of FIG. 26, humidity concentrations are not uniform. Furthermore, the humidity sensor (not shown) for the nitrogen purge controller may sometimes provide a misleading impression of the desiccator's overall humidity. Thus, the purge cycle may be discontinued prematurely. FIG. 27 shows the performance and concentrations of moisture within each chamber 1-5 shown in Figure A. The fluctuation shown in the graph of FIG. 27 seen in chamber 5 is a result of a controller unit (not shown) shutting on and off.
Figure B depicts a prior art, multi-chambered desiccator b where the humidity of each chamber 1-5 is purged with dry nitrogen gas N2 by sweeping the purge gas through individual chambers 1-5 in an attempt to achieve a laminar flow. In this embodiment humidity=concentrations are not uniform within the individual chambers. In order to maximize displacement efficiency, a perforated plenum chamber PC1 is utilized to provide a continuous, uniform gas flow to individual chambers. The chambers are connected in parallel with gas flow. They are not in communication with one another through their solid floors FL, and because of the positive pressure of the gas in the plenum chamber PC1, the gas does not flow between the chambers. Current state of the art designs utilize a door sensor DS (FIG. 6) on each door D of the chambers 1-5. The door sensor DS actuates a high-pressure purge whenever a door D is opened. The positive pressure within the chambers 1-5 inhibits moisture or contaminants from entering a chamber as gas flows out an open door D. For critical environment applications, multi-channel purge controllers are available, where each chamber has its own sensor and purge controller.
Failures Encountered by the Prior Art:
FAILURE TO DISPLACE: Typically, desiccators at lower humidity use a direct nitrogen purge to flush out moisture by displacement. However, displacement is an idealized concept. Where purging by displacement may be effective for piping or simple geometries, a desiccator cabinet with shelves stocked with content creates many dead end cavities. A laminar flow system also requires that gas be introduced uniformly across a broad area, creating a piston-like displacing force that is impossible to achieve in a desiccator, even one with a rear plenum wall. Because of this, the moisture-laden air is not effectively displaced. It is common that the purge gas plume can stream or arch from the inlet to the release valve and fails to displace or mix with moisture-laden air, resulting in a slow process.
UNEVEN GAS DISTRIBUTION: Even with perforated plenums, chambers still continue to receive unequal gas distribution, which inhibits rapid recovery times to achieve uniform humidity throughout all the chambers. This problem with prior art desiccators is illustrated by the graphs shown in FIGS. 26, 27 and 28.
SLOW AIRFLOW VELOCITY: Currently, the only applied method to reach dead end cavities is by increasing the velocity of gas plume thereby increasing the sweeping action. In conventional airflow systems, the sweeping action of the purge gas depends on the inlet velocity and direction, which are slow and inefficient, and on uniform gas exhaust, which requires multiple bleed valves that reduce internal positive pressure.                i. Gas flow of 50-200 Standard Cubic Feet per Hour (SCFH) distributed across multiple chambers does not provide adequate airflow to mix or uniformly displace induced content across the cross sectional area of the desiccator.        ii. The effectiveness of reaching dead end cavities depends on purge gas velocity and direction.        iii. Slow inlet velocity inhibits turbulence needed to mix purge gas with water vapor in dead-end cavities.        iv. Nitrogen gas is neutrally buoyant. This means that it is almost the same density as air.        v. Neutrally buoyant gases do not have any intrinsic movement of either up or down and therefore must be driven by an artificial air stream in order to be mixed quickly. And hence an ineffective medium to perform the sweeping action required by displacement at slow velocities.        vi. The lighter the purge gas, the more velocity is required to effectively mix with the induced content.        vii. Unassisted gas diffusion is slow and takes a long time to reach an equilibrium concentration.        viii. The turbulence created from the gas line air stream is not enough to assist the mixing of the water vapor dilution process within a large container.        ix. Slow gas velocity reduces the effectiveness of sweeping out moisture depends on the direction and velocity of the gas plume.        x. A high gas purge of 50-200 SCFH distributed across perforated plenum wall does not create an adequate gas flux for laminar purging.Type of Problems Encountered in the Prior Art:        
ACHIEVING SET POINT QUICKLY: End users frequently complain that nitrogen-purged desiccators do not achieve low-humidity set-points fast enough for moisture-sensitive content. Users must achieve set-point as fast and as efficiently as possible to either reduce wasteful gas consumption or to minimize the amount of time contents are exposed to moisture or oxygen. Unfortunately, the displacement model requires relatively high flow and therefore high gas consumption.
RECOVERY TIME: A major concern in nearly all industries is a desiccator's ability to recover the humidity set point quickly after a chamber is accessed (i.e., its door is opened and closed). Once the door is closed, users need the chamber to recover set point humidity as fast as possible. This leads to the problem outlined above.
KEEPING CONTAMINANTS OUT: This goes hand-in-hand with recovery time. When a user opens a door, moisture and particulate contaminants should be kept out.
UNIFORMITY: Current methods do not produce uniform humidity concentrations throughout the chambers, especially in single-channel control modules.
INTERRUPTION IN PURGE OPERATION: Stratification of gas or a plume of dry nitrogen gas can move over the humidity sensor, causing the sensor to measure low humidity concentrations and shut off the gas purge process prematurely. Consequently, this premature gas shut-off can compromise sensitive content within the desiccator.
ECONOMY OF USE: Current methods induce high levels of nitrogen gas consumption and waste to dilute moisture. This induces high consumption and waste of purge gas, driving up overhead cost. Wasteful purging can drive up overhead costs for manufactures with a higher frequency of nitrogen generator or nitrogen canister replacement. This is a common concern for users, particularly those who rely on gas canisters that must be frequently replaced.
Prior Art Solutions to Problems:
HIGHER PURGE RATE: The current solution to mitigate the problems stated above is to increase the gas flow rate and attempt to purge by brute force, but this solution increases gas consumption and operating expense.
PURGE BY DISPLACEMENT: Currently, the rationale behind the prior art design is to attempt to purge moisture by displacement. This means that nitrogen is released into the container without intermixing with the induced air and displacing it out of a release valve. In accord with the displacement model, designers install as many bleed valves as possible, as elaborated below.
PERFORATED PLENUMS: Perforated plenums attempt to improve uniformity and efficiency of displacing the air in the container with laminar air flow.
AUTOMATIC PURGE CONTROL UNITS: Purge control units switch to high purge when ambient is above set point and switch to a slow bleed purge after to maintain positive pressure. Additionally, some desiccators are configured with door sensors to actuate a high purge when a door is opened. This keeps contaminants out.
MULTIPLE BLEED/RELEASE VALVES: Multiple exhaust valves optimize purging efficiency and uniformity. The uniformity of the purge is proportional to the number of exhaust valves.
INDIVIDUALLY CONTROLLED CHAMBERS: The current most effective way to address economical consumption with a quick recovery time is to have a dedicated control unit and gas line for each individual chamber versus having one for all chambers in the cabinet. This can cost approximately $1,200.00 for each chamber and is typically too expensive for the market. Because of its price, there has been little commercial success compared to other configurations.